496 



LECTURE LII. 



ON THE MEASURES AND THE NATURE OF HEAT. 



THE principal particulars concerning the origin, the progress, and the 

 effects of heat, having been noticed in the last lecture, we now proceed to 

 examine the most usual modes of measuring its degrees and its quantity, 

 and to inquire into the most probable opinions respecting its intimate 

 nature and its immediate operation. 



The expansion of solids is measured by a pyrometer, which is calculated 

 for rendering the smallest change of dimensions perceptible either by 

 mechanical or by optical means. The first of these methods was adopted 

 by those who first investigated these effects ; a bar of metal being placed 

 in a vessel of water or of oil, which was heated by lamps, while the extre- 

 mities of the bar were in contact with a fixed point on one side, and on the 

 other with a series of levers, which multiplied the expansions so as to 

 render them easily observable by means of the end of the last lever, serving 

 as an index. But it is obvious that the expansion of the fixed part of the 

 instrument, and the irregular changes of temperature of the levers them- 

 selves, must very much interfere with the accuracy of such an instrument. 

 A much more correct mode of determination is to employ two microscopes, 

 fixed to an apparatus, which is always kept, by means of ice, at a constant 

 temperature, and to observe with a micrometer the change of place of 

 either end of the heated bar. 



For such purposes, the degrees of heat may be ascertained by the 

 natural measures of the freezing and boiling points of certain liquids, and 

 of water in particular ; but for subdividing the intervals between these 

 points, other means must be employed. The most natural mode of deter- 

 mining the intermediate degrees of heat, which must be considered as the 

 standard for the comparison of all others, is too laborious and complicated 

 for common use. If we mix together equal quantities of the same liquid 

 at two different temperatures, they will obviously acquire an intermediate 

 temperature, which is the natural mean between the separate temperatures, 

 provided that no heat be lost or gained during the process ; and provided 

 that no irregularity be produced from the approach of the liquid to a state 

 of congelation, the existence of which might be detected by a comparison of 

 experiments on various liquids at the same temperatures. By repeating 

 the operation, we may subdivide the intervals as often as we please, or we 

 may mix the liquids in any other proportion, so as to obtain at once any 

 other point of the scale, which may afterwards be identified by a thermo- 

 meter of any description. 



There is also another method of comparing the divisions of a thermometer 

 with those of the natural scale, but it is not wholly free from objections ; 

 the instrument being placed in a cone of the sun's rays, made to converge 

 by means of a lens or mirror, the quantity of heat falling on it must be 



